In a typical wireless communication system such as a cellular radio frequency (RF) radiotelephone system, a base station system having a controller and a plurality of transmitters and receivers communicates with a switching center and with a mobile communication unit, or mobile station, operating within an area served by the base station system.
Multiple access wireless communication between the base station and the mobile station occurs via RF channels which provide paths over which communication signals such as voice, data and video are transmitted. Base-to-mobile station communications are said to occur on a forward- or down-link channel, while mobile-to-base station communications are referred to as being on a reverse- or up-link channel.
Code division multiple access (CDMA) is one example of a well-known digital RF channelization technique. A communication system using CDMA channelization is described in detail in TIA/EIA Interim Standard IS-95A Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System, Telecommunications Industry Association, Washington, D.C. July 1993 IS-95A!; "TIA Telecommunications Systems Bulletin: Support for 14.4 kbps Data Rate and PCS Interaction for Wideband Spread Spectrum Cellular Systems", February 1996 the Bulletin!; and in TIA/EIA Interim Standard IS-96 IS-96!, IS-95A, the Bulletin and IS-96 incorporated herein by reference. Another communication system using CDMA channelization is described in TIA/EIA Interim Standard IS-99, Data Services Option Standard for Wideband Spread Spectrum Digital Cellular System, Telecommunications Industry Association, Washington, D.C., also incorporated herein by reference.
Typically, all users of a CDMA system transmit and receive communication signals over a common frequency spectrum, and specialized codes, such as Walsh codes, are used for separating multiple communication signals from one another in the system. Each specialized code typically represents one communication channel within the common frequency spectrum. A communication channel may be, among other things, a paging channel or a traffic channel.
IS-95A provides a maximum baseband data rate of 9600 bits per second for a communication signal on a particular down-link channel, although lower data rates are often used for periods of less speech activity. Generally, communication signals broadcast to subscriber units on traffic channels require different amounts of down-link power according to the baseband data rate, and a signal having higher baseband data rate is transmitted at a higher down-link power level than a signal having a lower baseband data rate.
It may be desirable in a CDMA communication system to deliver the same message to a number of mobile communication units. The message is commonly referred to as a broadcast message, and may be, among other things, traffic, weather or sports information, emergency information, speech or video segments or a software download.
It is generally known to deliver a broadcast message to a number of mobile communication units over a CDMA paging channel. See, for example, TIA/EIA Interim Standard IS-637, incorporated herein by reference. Because CDMA paging channels are generally broadcast with high down-link power, however, sending unnecessary or excessive messages over a paging channel may cause interference and may degrade down-link capacity. And down-link capacity may be further reduced if the broadcast message is transmitted over more than one paging code channel.
It is also known to deliver a broadcast message via a CDMA traffic channel. When a broadcast message is being transmitted over a traffic channel, it is typically sent at the maximum baseband data rate. When a large number of mobile communication units are to receive the message, a spike in down-link power may occur and cause poor reception at the mobile communication units.
Existing techniques for delivering broadcast messages over CDMA paging or traffic channels may also result in multiple copies of the same broadcast message in the common frequency spectrum received by each mobile communication unit.
There is therefore a need for an improved method for delivering a broadcast message over a CDMA signaling or traffic channel which reduces down-link interference and which minimizes a number of copies of the broadcast message in the common frequency spectrum.